This invention relates to a flowmeter for use in connection with metal pipes, and in particular a non-invasive flowmeter using a pulsed narrow band ultrasonic signal to detect different phases present in a fluid flowing through a pipe.
Flowmeters which are clamped externally onto pipes to measure flows occurring within the pipes in a non-invasive manner are well known. Certain of these flowmeters use an ultrasound signal to monitor single-phase flows in industrial pipes. However, these meters are limited in application as they cannot be used to monitor multi-phase flows of gas/liquid or gas/oil/water in metal pipes, such as are common in the oil industry. This is because the metal pipes cause unwanted reflection of the ultrasound signal and also gas bubbles in the fluid tend to attenuate and scatter the signal. This makes interpretation of a reflected ultrasound signal very difficult.
The present invention aims to provide a flowmeter which is capable of monitoring multi-phase flows occurring in metal pipes.
In accordance with one aspect of the present invention, there is provided apparatus for measuring flow rates in metal pipes, the apparatus comprising transceiver means generating a pulsed narrow band ultrasonic signal in the range 100 KHz to 10 MHz and matching means adapted to be coupled to a pipe so as to reduce or prevent reflections from occurring from a pipe wall in response to the pulsed signal. The use of a pulsed Doppler measurement in combination with a matching means allows analysis of multi-phase flows within metal pipes to be undertaken.
The pulsed signal is generated at a known frequency and as is usual with Doppler-type analysis, a shift in frequency of a signal returned from a moving body can be used to determine the velocity of the moving body.
Accordingly, the apparatus preferably further comprises a signal processing means for analysing the reflected signals received by the transceiver means. This allows shifts in frequency of the reflected signal from that of the generated pulsed signal to be analysed to give a measure of the velocity of phases within a flow contained in a metal pipe.
The signal processing means desirably further comprises means for calculating the energy contained in the Doppler frequency shifted part of the a reflected signal.
Preferably the signal processing means further comprises means for calculating flow rates of at least two phases in a flow contained in a metal pipe. The invention is particularly applicable to flows occurring in horizontal pipes and for two phase flows, although in certain circumstances analysis of flows containing three phases can be undertaken, as can analysis of flows in vertical pipes.
To allow an analysis of the reflected Doppler signals according to depth of origin within a pipe wall, the signal processing means may further comprise range-gate channels for acquisition of data relating to the reflected signals.
A number of different embodiments are possible for apparatus in accordance with the present invention, and thus the transceiver means may be a transceiver, the matching means an impedance matching device, with the transceiver and impedance matching device being contained in a common housing. Measurements using the apparatus are typically obtained at more than one position, and where the transceiver and impedance device are contained in the common housing, multi-positional measurements may be obtained by moving the apparatus around a pipe so that successive measurements are taken at different angles to the vertical.
In a further embodiment, the transceiver means preferably comprises a plurality of transceivers, and similarly the matching means may comprise a plurality of impedance devices, such that each transceiver may be held in a common housing with a respective impedance matching device. In such an embodiment, there are thus a plurality of housings each containing a transceiver and an impedance matching device and multi-positional measurement about the circumference of a pipe may be obtained by spacing the housings around an outer surface of the pipe.
Where a common housing is provided for a transceiver and an impedance device, preferably the housing includes a frequency transparent material to allow signals from the transceiver to be emitted from the housing without distortion. This ensures that the signal characteristics of the pulsed signal are not affected by the impedance device.
To ensure that there is sufficient contact area between the housing and a pipe on which measures of flow are to be undertaken, preferably the housing comprises a contact face for engaging with a pipe, the contact face having a groove of complimentary curvature to a pipe wall. This ensures that sufficient energy of the pulsed signal is transmitted into fluid contained in the pipe.
Desirably the impedance matching device has its impedance substantially matched to that of a pipe wall on which the apparatus is to be used. By matching the impedance in this way, energy transmission between the transceiver and the fluid is mainly limited to the region of direct contact between the transceiver and the pipe wall as dispersion of energy into the area of pipe surrounding the transceiver is limited due to damping associated with the impedance matching material.
The impedance matching device may comprise a sound absorbing block and the block may be made from plastics material, such as epoxy, with metal particles, such as tungsten, embedded therein. Rubber with tungsten particles embedded therein is also suitable, as is any material with an impedance similar to the metal from which the pipe is made.
The impedance matching device may also comprise an irregular surface to scatter unwanted reflective signals, and so minimise specular reflections. One type of irregular surface suitable, is a surface with a repeating saw tooth edge in profile.
The pulsed signal propagating in the pipe wall is preferably a shear wave, which again acts to limit the amount of energy dispersion along the length of the pipe, as opposed to the desired energy transmission through the pipe wall and into the fluid. Whilst an element of compressional wave may be within the pulsed signal, it is preferred that the pulsed signal propagating in the pipe wall is entirely a shear wave.
In accordance with another aspect of the present invention, there is provided a method for measuring flow rates in metal pipes, comprising attaching a transceiver means and matching means to a metal pipe containing fluids, generating a pulsed signal in the range 100 Khz to 10 MHz, transmitting the pulsed signal into fluid contained in the pipe, receiving a reflected signal from the fluid and analysing the reflected signal to determine different phases contained in the fluid.
The matching means reduces or prevents reflections occurring from a pipe wall in response to the pulsed signal.
Preferably, the method further comprises angling the transceiver means relative to a pipe to which it is attached, so as to achieve propagation of the pulsed signal as a shear wave. This helps ensure that a substantial proportion of the energy associated with the pulsed signal is transmitted into the fluid, and not just along the pipe wall.
The pulsed signal preferably propagates the shear wave at an angle between 35-85xc2x0 in a pipe wall, although other angles of incidence may be used, particularly where it is acceptable to have an element of compression wave in the pulsed signal. More preferably the angle should be between 45-75xc2x0.
The reflected signal is preferably analysed to determine its energy in the Doppler frequency shifted region, and analysis can also be undertaken using range-gate channels to allow the reflected signal to be analysed according to depth of origin within the fluid. Preferably the method further comprises measuring the reflected signal at a plurality of positions, the measurements being spaced apart in time. This can either be achieved by moving the transceiver means and matching means around the metal pipe, or by the transceiver means and matching means comprising a plurality of pairs of transceivers and impedance devices, these pairs being spaced apart on the pipe surface to achieve multi-positional measurement.